Picasa is an excellent image organizer in general, but its integration with Google Maps and Earth make it easy to add location information to the metadata of your photos. If you're not already using it, it's a free download as is Google Earth, which you'll need to install as well if you want to display photos on its satellite map.
In this example, I'm using some pictures taken of the Empire State Building. Find and select the photos being tagged and in the lower right corner of the interface, click the Places button. Places uses Google Maps to get the coordinates for your photos.
When you've found the location you'd like to associate with your photos, Picasa will open a dialogue box asking you to confirm that it's the correct spot. In this case, I selected six photos of the Empire State Building to be geotagged.
After an image is tagged, a small red pin appears in the lower right corner of your photo. Right-click on the photo and select Properties at the bottom of the pop-up menu. This will show you all of the metadata the image is tagged with, including the longitude and latitude information.
Picasa also features a geotag button in its interface. However, instead of simply tagging photos with location data, it also places them on the Google Earth globe. Again, you must have Google Earth installed on your computer to use this feature.
Unfortunately, this isn't available for Mac users. There is a work-around, though. Once your images are geotagged--through Google Maps or another method--you can use a free iPhoto plug-in to create a KMZ file viewable in Google Earth.
When you click the geotag button in Picasa (after you've selected the images you want to tag, of course), Google Earth will launch and ask you to navigate to the spot where you'd like to place your photos. You can do it manually using the crosshair (not shown)and directional controls in the upper right of the interface. Or, in the upper left, is a search bar.
After you've navigated to the correct location, you simply click Geotag and Earth will add the metadata and place the photos on the Google Earth globe.